Installer Changes

During the setup the "sass" gem is installed. This works fine on Linux operating systems like Ubuntu. However, on some other distributions such as Arch Linux "gem install" defaults to the a local folder in the users home directory. When installed here the sass binary is not in the path and coco-brunch cannot find it (and thus crashes with an error). By adding the "--no-user-install" flag we are telling gem to install it globally no matter what the system default is, fixing this problem.